1. Field of the Invention
The invention relates to a flexible multilayer printed circuit assembly, and in particular, to a flexible multilayer printed circuit assembly that reduces EMI emissions by incorporating a shielding fence, a small via pad, or both.
2. Background Information
As processing speeds and packaging densities have increased, traditional wire cables cannot meet the requirements of high-speed signal interconnections. Flexible multilayer printed circuit assemblies have been developed for the high-speed interconnections between multiple computer processing units. These flexible multilayer printed circuit assemblies are high circuit density systems that can save space, can handle multiple conductive layers with its high circuit density, and can provide high-speed interconnections between multiple units. Because its flexibility greater than rigid printed circuit assembly, the flexible printed circuit assembly may be used where the assembly should be bent to interconnect computer units together. These benefits have increased use of the flexible multilayer printed circuit assemblies in place of the traditional wire cables for computer processor interconnections.
A typical flexible multilayer printed circuit assembly has multiple conductive layers that are made of conductive material. Each conductive layer is separated by a dielectric material. Generally, the conductive layers include logic ground plane layers used to provide a reference voltage plane or a logic ground plane. The conductive layers also include one or more signal wiring layers used to provide high-speed interconnect signal wirings. The signal wiring layer is sandwiched between the logic ground plane layers.
Information Technology Equipment (ITE) such as computers should meet Electromagnetic Interference (EMI) requirements for many countries. EMI emissions from the ITE equipment should be below CISPR 22 and FCC limits while the ITE equipment is operating in a typical manner. Accordingly, the conductive layers further include shield plane layers to shield the EMI emissions. In order to contain the EMI emissions coming from the high-speed interconnect signal wirings and electrical noise on the logic ground plane layers, the shield plane layers are formed as a top layer and a bottom layer of the flexible multilayer printed circuit assembly.
The typical flexible multilayer printed circuit assembly further includes vias to provide layer-to-layer interconnections. For example, the logic ground planes should be connected together using ground vias to keep them at the same potential. The top and bottom shield plane layers are connected to each other using shield vias along the perimeter of the structure to provide an electrical and mechanical connection between them.
Referring to FIG. 1, a conventional flexible multilayer printed circuit assembly 10 is provided. The printed circuit assembly 10 includes a top shield plane layer 11, a first logic ground plane layer 12, a first signal wiring layer 13, a second logic ground plane layer 14, a second signal wiring layer 15, a third logic ground plane layer 16, and a bottom shield plane layer 17. Each of the conductive layers 11 to 17 are separated from each other by layers made of a dielectric material. The printed circuit assembly 10 further includes logic ground vias 18 that electrically connect the logic ground plane layers 12, 14 and 16 together, and shield vias 19 that mechanically stitch and electrically connect the shield plane layers 11 and 17 together. The logic ground vias 18 may be formed as buried vias that extend between the ground plane layers. However, because such buried vias require higher manufacturing cost, the logic vias may be also formed as through-hole type vias, which penetrate the flexible multilayer assemblies from a top and a bottom thereof, as shown in FIG. 1. It is noted that the logic ground vias are separated from the shield plane layers 11 and 17, although they extends from a top surface and a bottom surface of the flexible multilayer printed circuit assembly 10, as shown FIGS. 1 and 2.
Unlike the traditional wire cables, flexible multilayer printed circuit assemblies cannot be easily shielded using metal foil and metal braids that may surround all of the top, bottom, and side surfaces of the assemblies. Because the signal wiring layers 13 and 15 and logic plane layers 12, 14 and 16 are electrically noisy, these layers 12, 13, 14, 15 and 16 should be contained inside a shield structure so as to contain the EMI emissions from those layers. As shown in FIG. 1, only top and bottom surfaces of the assembly 10 are shielded by the top and bottom shield plane layers 11 and 17. As a result, the EMI emissions in a direction toward the top and bottom shield plane layers 11 and 17 are shielded by the top and bottom shield plane layers 11 and 17. However, the EMI emissions of the flexible multilayer printed circuit assembly 10 in other directions, e.g., a horizontal direction to which the signal wiring layers 13 and 15 and the logic plane layers 12, 14 and 16 extend, are not sufficiently shielded. Thus, the flexible multilayer printed circuit assembly 10 will yield EMI emissions from the logic ground vias 18 protruding through the assembly surfaces and in proximity to the assembly surfaces, such as the edges of the assembly 10. These individual emissions can add together and potentially cause the ITE equipment to fail the EMI requirements.
Referring to FIG. 2, the shield vias 19 include top via pads 192 formed in the first shield plane layer 11 and bottom via pads (not shown) formed in the second shield plane layer 17. The logic ground vias 18 also include top via pads 182 and bottom via pads (not shown) respectively formed in the first and second shield plane layers 11 and 17. Conventionally, the top and bottom via pads of the logic ground vias 18 are as big as the top and bottom via pads of the shield vias 19, e.g., in diameter. Because a bigger logic ground via pad yields higher EMI emissions, the conventional printed circuit assembly with a big-sized logic ground via pads does not sufficiently reduce the EMI emissions.